Most of you will wonder why there will be only only 21 million Bitcoins in existence? Why not some other number? 1 million or 1 billion let's say. Ηow this amount of Bitcoins was decided?
Remember the number. 21 million Bitcoins!

First of all,, let's quickly see what a "hard cap" or maximum supply is, and the important role it has in value of a cryptocurrency, in our case in Bitcoin price.
The maximum supply of a cryptocurrency can not change and is the maximum number of coins that will ever be in existense. There can't be more coins than that, as it is the total amount can ever be mined or produced.

The creator of Bitcoin, Satoshi Nakamoto, thought about maximum number of Bitcoins long enough before deciding and I don't think that it is just a random number.
Calculate the number of blocks per cycle every four year:
6 blocks per hour
* 24 hours per day
* 365 days per year
* 4 years per cycle
= 210,240
~ 210,000
Sum up all the block reward sizes considering halvings:
50 + 25 + 12.5 + 6.25 + 3.125 + … = 100
Multiply the two:
210,000 * 100 = 21 million.